diff --git a/lib/Db/DbDumper.php b/lib/Db/DbDumper.php
index 2202a4c..841f9d0 100644
--- a/lib/Db/DbDumper.php
+++ b/lib/Db/DbDumper.php
@@ -72,6 +72,9 @@ final class DbDumper {
 	public function dumpStep(Repository $repo, array $state, float $deadline, array &$stats): array {
 		$state['steps']++;
 		$sm = $this->db->createSchemaManager();
+		// A pack that fills up during the dump is uploaded right away; its index cache row must
+		// wait until the read-only transaction is over.
+		$repo->holdIndexCache();
 		$this->db->setTransactionIsolation(TransactionIsolationLevel::REPEATABLE_READ);
 		$this->db->beginTransaction();
 		try {
@@ -116,6 +119,7 @@ final class DbDumper {
 			}
 		} finally {
 			$this->db->rollBack();
+			$repo->releaseIndexCache();
 		}
 		$repo->flushPacks();
 		return $state;
diff --git a/lib/Repository/BlobIndex.php b/lib/Repository/BlobIndex.php
index cae45cf..8831e49 100644
--- a/lib/Repository/BlobIndex.php
+++ b/lib/Repository/BlobIndex.php
@@ -25,6 +25,8 @@ final class BlobIndex {
 	private array $pending = [];
 
 	private ?IIndexCache $cache = null;
+	/** @var list<\Closure():void>|null cache writes held back by holdCache(), or null when writing directly */
+	private ?array $heldCacheWrites = null;
 	private string $repositoryId = '';
 	/** @var int index files downloaded by the last load() (for tests and diagnostics) */
 	public int $downloaded = 0;
@@ -40,6 +42,46 @@ final class BlobIndex {
 		$this->repositoryId = $repositoryId;
 	}
 
+	/**
+	 * Hold back cache writes until releaseCache(). The database dump reads inside a read-only
+	 * transaction on the same connection that the cache writes to: a write there fails on
+	 * PostgreSQL and would be rolled back with the dump transaction elsewhere.
+	 */
+	public function holdCache(): void {
+		$this->heldCacheWrites ??= [];
+	}
+
+	/** Write the cache changes held back since holdCache(). */
+	public function releaseCache(): void {
+		$writes = $this->heldCacheWrites ?? [];
+		$this->heldCacheWrites = null;
+		foreach ($writes as $write) {
+			$write();
+		}
+	}
+
+	/** @param list<array{id:string, offset:int, length:int, raw:int, flags:int}> $entries */
+	private function cachePut(string $packId, array $entries): void {
+		if ($this->heldCacheWrites !== null) {
+			$this->heldCacheWrites[] = function () use ($packId, $entries): void {
+				$this->cache?->put($this->repositoryId, $packId, $entries);
+			};
+			return;
+		}
+		$this->cache?->put($this->repositoryId, $packId, $entries);
+	}
+
+	/** @param list<string> $packIds */
+	private function cacheRemove(array $packIds): void {
+		if ($this->heldCacheWrites !== null) {
+			$this->heldCacheWrites[] = function () use ($packIds): void {
+				$this->cache?->remove($this->repositoryId, $packIds);
+			};
+			return;
+		}
+		$this->cache?->remove($this->repositoryId, $packIds);
+	}
+
 	/**
 	 * Load the index: one listing of index/, entries from the local cache where available,
 	 * and only the index files that are not cached yet are downloaded. Cached packs that no
@@ -112,7 +154,7 @@ final class BlobIndex {
 		fwrite($stream, $this->cipher->encryptString(json_encode($entries, JSON_THROW_ON_ERROR), 'index:' . $packId));
 		rewind($stream);
 		$this->backend->put('index/' . $packId, $stream);
-		$this->cache?->put($this->repositoryId, $packId, $entries);
+		$this->cachePut($packId, $entries);
 		foreach ($entries as $e) {
 			$this->blobs[$e['id']] = [$packId, $e['offset'], $e['length'], $e['raw'], $e['flags']];
 			unset($this->pending[$e['id']]);
@@ -131,7 +173,7 @@ final class BlobIndex {
 	/** Forget a pack: delete its index file and cache row. Blobs that now live elsewhere are kept. */
 	public function removePack(string $packId): void {
 		$this->backend->delete('index/' . $packId);
-		$this->cache?->remove($this->repositoryId, [$packId]);
+		$this->cacheRemove([$packId]);
 		foreach ($this->blobs as $id => $loc) {
 			if ($loc[0] === $packId) {
 				unset($this->blobs[$id]);
diff --git a/lib/Repository/Repository.php b/lib/Repository/Repository.php
index 9a383ae..8c98ce3 100644
--- a/lib/Repository/Repository.php
+++ b/lib/Repository/Repository.php
@@ -262,6 +262,15 @@ final class Repository {
 		});
 	}
 
+	/** See BlobIndex::holdCache(): no database writes until releaseIndexCache(). */
+	public function holdIndexCache(): void {
+		$this->index->holdCache();
+	}
+
+	public function releaseIndexCache(): void {
+		$this->index->releaseCache();
+	}
+
 	public function flushPacks(): void {
 		$this->runPacks?->flush();
 	}
diff --git a/tests/Unit/Repository/IndexCacheTest.php b/tests/Unit/Repository/IndexCacheTest.php
index c2b0c5b..73ecdd3 100644
--- a/tests/Unit/Repository/IndexCacheTest.php
+++ b/tests/Unit/Repository/IndexCacheTest.php
@@ -51,4 +51,41 @@ class IndexCacheTest extends TestCase {
 		$fourth->restore($r1['snapshot'], $target);
 		$this->assertCount(12, self::hashTree($target));
 	}
+
+	/** The database dump reads in a read-only transaction: a pack that fills up then must not write the cache yet. */
+	public function testHeldCacheWritesWaitForRelease(): void {
+		$cache = new class implements IIndexCache {
+			public array $data = [];
+			public bool $readOnly = false;
+			public function all(string $r): array { return $this->data[$r] ?? []; }
+			public function put(string $r, string $p, array $e): void {
+				if ($this->readOnly) {
+					throw new \RuntimeException('cannot execute INSERT in a read-only transaction');
+				}
+				$this->data[$r][$p] = $e;
+			}
+			public function remove(string $r, array $ps): void { foreach ($ps as $p) { unset($this->data[$r][$p]); } }
+		};
+		$backend = new LocalBackend($this->tempDir());
+		$keys = KeyRing::generate();
+		Repository::initWithKey($backend, $keys, $keys->wrap('pw'));
+		$repo = Repository::openWithKey($backend, $keys, $cache);
+
+		$stats = [];
+		$repo->holdIndexCache();
+		$cache->readOnly = true;
+		$writer = $repo->blobWriter($stats);
+		for ($i = 0; $i < 40; $i++) {
+			$writer->write(random_bytes(1048576)); // more than one 32 MiB pack
+		}
+		$writer->finish();
+		$this->assertGreaterThan(0, count($backend->list('index')), 'a full pack was uploaded during the hold');
+		$this->assertSame([], $cache->data);
+
+		$cache->readOnly = false;
+		$repo->releaseIndexCache();
+		$repo->flushPacks();
+		$this->assertCount(count($backend->list('index')), array_merge(...array_values($cache->data)));
+		$this->assertSame(0, Repository::openWithKey($backend, $keys, $cache)->indexDownloads());
+	}
 }